As a primary concern for human health, bone fractures are associated with poor bone mineralization during growth spurts and increased risk of osteoporosis in the elderly(1,2). Osteoporosis causes bone fractures by lowering bone mass and deteriorating bone architecture(3). Hip fractures alone are estimated to reach >6 million cases a year by 2050(4). While osteoporotic fractures occur mostly in people over the age of 70(5), enhancing peak bone mass early in life may postpone or prevent the incidents(6,7).
Osteoporosis is a major public health problem(49) afflicting at least 10 million people in the US(50). Although a number of drugs are available to treat this disorder, most of these drugs act to only inhibit bone resorption(51,52,53). Because bone metabolism depends on the balance between the osteoblasts-mediated formation and the osteoclasts-mediated resorption(54), effectiveness of these drugs is fairly limited in restoring bone integrity(55,56). In contrast, maximizing bone mass and strength at an early stage of life offers the most effective strategy to prevent or alleviate osteoporosis at later life stages(57,58). Nutritional manipulation is considered to be one of the major exogenous factors to enhance peak bone mass at early life stages(59,60,61).
Mostly, only surrogates are available for in vivo bone property and function assessments in humans(7). Thus, animal models offer an advantage for the actual measurements of biophysical characteristics and chemical compositions of bones. Among several small and large animal models(8), canine and porcine bones resemble human bones in many features including density and stress fracture properties(9). Because of implications of estrogen in the occurrence of osteoporosis for women(10) and similarities of the pig estrus cycle to the human menstrual cycle(11), pigs seem to be a better model than dogs for human osteoporosis research.
Microbial phytase has been widely used during the past decade as a feed additive for swine to enhance utilization of phytate-P from plant feeds(12). Studies have shown effectiveness of the enzyme in replacing inorganic P supplementation to support normal growth performance and bone strength of pigs fed low-P diets(13,14). The enzyme releases P and other chelated elements including Ca, Fe, Zn, Mn, and Cu for absorption in the gastrointestinal tract, allowing possible incorporations of these elements into bone(15,16). A few experiments(17,18) have shown potential benefits of dietary phytase to bone properties in pigs fed P-adequate diets. Because these experiments were conducted to optimize growth and production responses of pigs, data on bone responses of pigs from these studies offered limited implications for human bone health issues.